1. Field of the Invention
The present invention relates to an exhaust emission control device and method for an internal combustion engine, and an engine control unit, for purifying i.e. decreasing exhaust emissions by temporarily trapping NOx contained in exhaust gases emitted from the engine and performing reduction of the trapped NOx.
2. Description of the Related Art
Conventionally, there has been disclosed an exhaust emission control device for an internal combustion engine, e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2006-207487. This internal combustion engine is a gasoline engine, in which the lean operation in which the air-fuel ratio of a mixture is controlled to a leaner value than a stoichiometric fuel-air ratio is carried out depending on operating conditions of the engine. Further, the exhaust emission control device is provided with a NOx catalyst which is disposed in an exhaust pipe for purifying NOx contained in exhaust gases. In the exhaust emission control device, NOx contained in exhaust gases emitted during the lean operation is trapped in the NOx catalyst. Further, when the amount of the trapped NOx exceeds a predetermined value, a rich spike is executed to enrich the air-fuel ratio of the mixture, whereby unburned components of exhaust gases are supplied to the NOx catalyst as a reducing agent to perform reduction of NOx to thereby purify NOx.
Further, values of the amount of NOx absorbed in the NOx catalyst during the lean operation and the amount of NOx purified and thereby eliminated from the NOx catalyst during execution of the rich spike are set in advance according to operating conditions of the engine (the intake air amount and the load on the engine), and are stored in respective maps. Then, a value of the amount of the absorbed NOx and a value of the amount of the eliminated NOx are read from the respective maps during operation of the engine according to detected operating conditions thereof, and an amount of NOx trapped in the NOx catalyst is estimated by subtracting the read value of the amount of the eliminated NOx from the read value of the amount of the absorbed NOx.
However, when the exhaust emission control device has a three-way catalyst disposed on an upstream side of the NOx catalyst in addition to the NOx catalyst, the three-way catalyst as well occludes and performs reduction of part of NOx contained in exhaust gases, which reduces the amount of NOx actually flowing into the NOx catalyst. Further, during execution of the rich spike, part of NOx having flowed into the NOx catalyst sometimes actually passes through the NOx catalyst without being trapped therein. In such a case, the actual amount of NOx trapped in the NOx catalyst becomes smaller.
In the conventional exhaust emission control device, however, the amount of trapped NOx is estimated only according to the operating conditions of the engine, as described above, which makes it impossible to accurately estimate the amount of NOx actually trapped in the NOx catalyst. Therefore, when the amount of trapped NOx is estimated to be smaller than the actual amount, timing for starting the rich spike, which is set according to the estimated value of the trapped NOx amount, is retarded. This causes insufficient reduction of NOx, resulting in increased exhaust emissions. Inversely, when the amount of trapped NOx is estimated to be larger than the actual amount, the timing for execution of the rich spike is advanced to cause supply of an extra amount of reducing agent. This causes excess amounts of CO and HC to be generated, resulting in increased exhaust emissions and degraded fuel economy.